The present invention relates generally to cutting glass and, more particularly, to an apparatus and method for cutting elongated strips from flat glass in ribbon or sheet form.
A common expedient for cutting glass is to score the major surface of the glass along the intended line of cut and then apply a bending moment across the score line to snap or fracture the glass therealong. However, it is difficult to achieve a quality cut when cutting elongated strips, such as undesirable selvedge portions, from thick glass bodies. As the thickness of the glass increases, more difficulty is encountered in obtaining a true and clean break along the score lines. Very often, the cut leaves rough, jagged or otherwise irregular severed edges as well as edges that are non-perpendicular to the major surfaces of the glass. It is believed that the reason for this is that a residual, unbalanced stress pattern is set up in the glass, the stress varying across the width of the sheet or ribbon due to conditions inherent in the formation of thick glass. One of these adverse conditions may be the nonuniform cooling across the width of the glass body. As a result, an area of high residual compressive stresses is produced along the outer edges of the ribbon after it has cooled below the annealing temperature range. Accordingly, when running a cut longitudinally of the ribbon within this area in an effort to trim the selvedge edge, the selvedge distorts on being separated from the main body to cause edge damage as well as sometimes severely fragmenting the cut-off portion or selvedge.
One solution to this problem involves redistributing the stress pattern within the glass body prior to making the final cut. This is effected by removing a portion of the glass along the outer edges thereof outwardly of the final or intended line of cut prior to making such final trim cut. This preliminary cut reduces the residual compressive stresses in this region of the ribbon and significantly relieves the compressive stresses along the final intended line of cut to produce a more uniform stress pattern therealong. The resultant cut along such final or intended line of cut produces a severed edge that is clean and smooth and substantially free from defects.
Attempts to make the first trim cut by apparatus for automatically applying a bending moment along the score line is not entirely satisfactory because the high degree of compression in this region of the glass often causes the cut to deviate laterally from the first score line, the cut sometimes meandering inwardly across the final intended line of cut. Also, the inherent high compressive stresses residing in the region of the first score line can cause the glass to fragment when a cut begins to run, posing a safety hazard to the operators or attendants of the apparatus. Consequently, it has been found preferable to make the first trim cut manually by utilizing nipping tools or pliers. However, it can be appreciated that the manual labor involved in such a continuous cutting operation adds materially to production costs.